In a system of this type, used for example in determining the distance of a vehicle from an observation point, it is known to provide the master station with a first pulse transmitter for sending an interrogation pulse to the slave station and with a first pulse receiver for intercepting a reply pulse sent back from the slave station. The latter, in turn, comprises a second pulse receiver for intercepting the interrogation pulse and a second pulse transmitter triggerable by the second pulse receiver, with a certain delay, for emitting the reply pulse. The delay, of predetermined duration, is designed to prevent interferences between the trailing end of the interrogation pulse arriving at the slave station and the leading end of the outgoing reply pulse. At the master station, this delay is taken into account by starting the measurement of the transit time between the two stations only after timing a like delay from the emission of the interrogation pulse; this latter delay serves to provide a guard interval during which the receiver of the master station may be made insensitive to incoming pulses which could be spurious reflections from nearby objects. An accurate determination of distance requires, of course, that the delay introduced at the slave station be practically constant and that its magnitude be precisely known at the master station.
In practice, however, deviations from the presumed duration of the delay period frequently occur for various reasons including ambient conditions such as temperature, aging of components, or manufacturing tolerances.